1. Field of the Invention
The present invention relates to a clock generating circuit. The present invention particularly relates to a clock generating circuit that generates a clock signal depending on an output voltage from a battery, and can steadily generate a clock signal of a desirable frequency even if the output voltage fluctuates due to a discharging characteristic of the battery. The present invention also relates to a semiconductor device provided with the clock generating circuit.
2. Description of the Related Art
Various electronic appliances have spread, and a variety of products is shipped to the market. The spread of portable electronic appliances is particularly notable. For example, the resolutions of the display portions and decay durability of the batteries have been improved for portable telephones, digital cameras, and the like, and their power consumption have been reduced, which makes them more convenient. Power for driving a portable electronic appliance is secured by a battery that is a storage means, which is incorporated in a structure of the portable electronic appliance. As the battery, a battery such as a lithium ion battery is used.
As an example of a portable electronic appliance, research and development for practical application of RFID (radio frequency identification) tags (hereinafter called “RF tags”) have been actively pursued. RF tags can be classified into two types: an active type and a passive type. The active type includes a battery and can transmit an electromagnetic wave containing information of an RF tag. Also, separately from the active type RF tag, the development of a technique for charging a battery for securing power for driving an RF tag using electromagnetic wave generated from a portable phone or the like (see Patent Document 1: Japanese Published Patent Application No. 2003-6592) has been in progress.
Note that an RF tag is also called an IC (integrated circuit) tag, an IC chip, an RF chip, a wireless tag, or an electronic tag.
On the other hand, since a discharging characteristic of a battery is degraded over time, the development of a technique for efficiently utilizing a remaining amount of power in the battery has been in progress (see Patent Document 2: Japanese Published Patent Application No. 2003-69891). This is done by detecting the remaining amount of power in the battery when an output voltage of the battery becomes lower, and having a plurality of modes for changing a driving frequency for driving an electronic appliance.